Add utility class Position.

Position represents cursor position in a buffer. Adapted from UltiSnips.
Add cursor property to Buffer.
Add partial_xml_parser utility.

Also added test version of 'gf' mapping for launch files.
Add map() function to vimp.
Add xacro filetype with 'gf' mapping.
Update Syntastic integration for launch files.
Extend vimp.Buffer class.

* Added Position-based element access with slicing support.
* Added iterator over characters in the buffer.
Add vimp.syntax module.
Add 'gd' mapping for msg, srv, and action files.

Put the cursor over a message type name and type 'gd' in normal mode to
open the definition of the message type.